Process for production of cellular thermoplastic products



Feb. 23, 1954 J MCCURDY ET AL 2,669,751

PROCESS FOR PRODUCTION OF CELLULAR THERMOPLASTIC PRODUCTS Filed June 29, 1950 t INVENTORS. I. JOAn-L/QyOMCCMr y BY C/mncey Eaworael any ATTORNEYS Patented Feb. 23, 1954 UNITED STATES,

PATENT OFFICE PROCESS FOR PRODUCTION OF CELLULAR THERMOPLASTIC PRODUCTS Applicationdune 29 1950, Serial No, 171,148.

'7 Claims.

This invention concerns an improved process for the manufacture of cellular thermoplastic products, It relates more particularly to the pr duc on of such. products in the form of substantially uniform cellular bodies in continuous manner from the thermoplastic benzene-soluble polymers and copolymers of monovinyl aromatic compounds having the vinylradical directly attaghed toacarbonatom of the aromatic nucleus, periifiularl o ys yre e,

McIntirein United States Patent 2,450, l36 has disclosed a methodfor the preparation of cellulair thermoplastic products which involves maintaining a, solidthermoolastic resin, e. g polystyrene, and a normally gaseous agent such as methyl chloiide, methyl ether, propylene or butylene, in a closed vessel under pressure at a temperaturebelow the critical temperature of the normally gaseous agent until a homogeneous mobile gel is obtained and, thereafter opening an, outletto. permit fiQW of thegeljrom the vessel. During flow of the mobile gelfrom the pressurized vessel intoa zone oflovver pressure, the resin is swollen, by vaporization and expans on of the dissolved volatile substance to form a stable cellular product consisting for the most part of individually-closed thin-walled cells.

A, methodofiorming under pressure, a mixtnre o'fa predetermined proportion of a normally gaseous agent and a thermo lastic resin and storingthe mintureby feeding the same into a pressurized storage vessel wherein it is maintainedata desired temperatur until ahomogeneous mobile gel or solution is obtained, prior to extrusion and expansion of the resin, as .iust mentioned, is des ribed in application Perial No. 78,4 ,518, filed November '7, 1947, by 0.3. McIntire, noW1Patent l lo.'2,5l5,25

The meth d heretofor proposed forthe manufacture of celu ar thermoplastic, products by batchwise flow of, a mobile gel of a normally gaseou agent and a thermo lastic resin under pressure from a closed vessel into a zone of lower pressure, is lessflexible in operation than is desiredfor many purposes. For instance, the operations of dissolving, under pressure in a closed vessel, apredetermined proportionof a normally gaseous agent in a thermoplastic polymer at a temperature suitable for the extrusion, or of form ng the gel at a different temperatur and, ita tr he it s d t e ature, are

difiicult and timeeconsuminswe. g. it frequently 1 ,aystolatyeek ordertonarry H tons; also, d n ng 'tusionof the resultant gel the latter tends to cling;

Walls of the vesseland funnel downward sothat remaining gel can bedischarged satisfactorily by, immediately closing the, extrusion valve when gas escapes, letting, the system stand to permit drainage of gel from inner walls of. the vessel and again, opening the valve. However, care must be o served during the extrusion and also during recharging of the vessel that the vapor pressure insideithe vessel does not decrease sufl'lcientlyto permit appreciable expansion to a cellular mass of the gel remaining in the vessel. If these precautions are not observed, extrasteps for removal of the cellular material maybe required in order to permit satisfactory re-employmentof the vess l in the process.

Also, the use of astorage vessel, or other, device, e. g. aheat exchanger, to condition or age a mixture, orbody, of a normally gaseous agent anda thermoplastic polymer so as to obtain a homogeneous mobil gel having ,a uniform temperature throughout its mass, prior to extrusion of the same to form a cellular product, does not al ow as great a flexibility in the operation of the processes is desired, particularly with regard to rapidly effecting changes, in the composition, or the temperature, of the solution being expanded to form the, cellular product.

It will be evident from the foregoing that manufacture of the cellularmasses would be sim plified and rendered less time-consuming and less costly by provision of a method for rapidly blending under pressure and bringing to a desired tem eraturein continuous manner. a pastic mix to form a homogeneous mobile gel or so t on of a pre etermned p oport n o a normallv gaseo s agent in a thermoplastic polvr er and extrudin the ge intn a one 0* ower oressure as it is formed. It is an object of the invention to providesuch a metho and also an apparatus permitting pract ce of the method in a continuous manner which avoids the need of storing or otherwise pre-conditioning the solution or gel for prolonged periods of time, prior to extrusion of the, same and expansionof the pol mer toform a cellular mass. Another ob,-

jectis to provide a, method for rapidly blending undenpressurea plastic mix of a normally gaseousagent and a thermoplastic polymerto a state of homogeneity ,and bringing the mixture to. {a desired temperature in continuous manner. Still 'anot er. object is tol r nces, methodfor pm;-

:a substantially homogeneous illobile' get or solution of a predetermined proportion of a normally gaseous agent in a thermoplastic polymer under pressure and for expansion of the gel as it is formed to produce a uniform cellular mass of the polymer. Other and related objects will become apparent from the following description of the invention.

According to the invention a cellular thermoplastic product com osed for the most part of individuallywlosed, thin-walled cells of substantially uniform size, can readily be prepared in continuous manner by forming a homogeneous mobile gel or solution of a normally gaseous agent in a thermoplastic polymer under pressure. which gel or solution is rapidly brought to a substantially uniform composition and temperature throughout its mass at the point of extrusion. and thereafter extruding the mobile gel or solution through a constricted passageway. e. g. an extrusion die or orifice, into a zone of lower pressure as it is formed.

It has been found that a homogeneous mobile gel or solution of a normallygaseous agent in a the moplastic rolymer. wh ch e or solut on has a substantially uniform temperature and composit on throughout its mass at the point of extrusion. can readily be obtained by feeding a thermop ast c polymer, e. g. rolystyrene, in a heatastified or molten, condition, preferably the atterv and a norma ly gaseous a ent such as methyl chloride. methyl ether, pro ylene. or butvene. into adm xture with each other under pressure in a mixing and cooling zone, wherein the mixture is caused to flow throu h said zone by pressure of the materia s fed thereto, a itating the mixture under pressure transverse y to the direction of flow thereof through said zone to intimatel blend the normally gaseous agent with the polymer while bringing the mixture to a un form temperature at which it rema ns fiowable and be ow the boiling point of the normally gaseous agent at the pressure thus ap lied to the mixture. The mobile gel or solution in a state of homo ene ty is ex anded as it is formed by extruding or discharging, the same from the mixing and cooling zone throu h a con-- stricted passageway such as an extrusion die or orifice into a zone of lower pressure, e. g. into the atmosphere.

The invention will be illustrated with reference to the accompanying drawing, partly in section. show ng one of the various forms of appararus which may be used in practicing the invent on, wherein Fig. 1 is a longitudinal section through the barrel of a plastics extruder in combination with a mixing and cooling device and an extrusion die or orifice; and

Fig. 2 is a cross-sectional view of the mixing and cooling device taken along the line 2-2 of Fig. 1.

In the drawing, the numeral 3 desi nates a feed screw of the compression type in the cylindrical barrel 4 of a horizontal plastics extrusion machine 5. Screw 3 usually has a perforated seal ng plate. or ring 0, on a mid-section of the screw, suitably from one-half to three-quarters of the distance from the feed hopper to the discharge end of the screw. Such sealing plate or ring is usually an integral part of the screw 3. Sealing plate or ring is suitably an annular enlar ement of the body of the screw 3, of a consist of a perforated plate secured to the screw 3 and containing a number of drill holes of from 0.125 to 0.375 inch diameter, in which case a clearance of from 0.010 to 0.020 inch between the bore of the barrel 6 and the rim of the sealing plate 0, is satisfactory. The barrel 4 is surrounded by chambers l' and la for heating or cooling, as required. Beyond sealing plate 6 on the screw 3 is an inlet 8 into the barrel d of the extruder. Head 9 is secured to the body of the extruder 5 as shown. Head 0 is also connected to an inlet ill at one end of a horizontal pressure resistant cylindrical mixing and cooling vessel H. The vessel H is provided with inlets i0 and i2 and outlet i3, which latter has attached thereto extrusion die M. A hollow roll iii of a diameter enough smaller than the bore of the cylindrical vessel M to leave a clearance of from 0.25 to 0.75 inch or more, between the two members is rotatably mounted within the vessel H with running clearance between the ends of the roll and the ends of the vessel H. The roll l5 suitably has a number of lugs or projections [6 secured to its outer surface and adapted to leave a clearance of from 0.010 to 0.020 inch between their ends and the bore of the mixing and cooling chamber H. The vessel Ii is surrounded by chamber l! for heating or cooling as required. The hollow roll Ill is provided with inlet I8 and out et It! for passing heating or cooling fluid into contact with inner surfaces of the roll l5, as required. Roll I5 is driven by any usual means such as an electric motor and variable speed drive (not shown) connected to shaft 20.

The essent al feature of the invention is the uniform b ending and transfer of heat throughout a mixture of a heat-plastified or molten thermoplastic polymer and a normally gaseous agent under pressure, which occurs in the mixing and coo ing vessel to produce rapidly a homogeneous mobile gel or solution having a substantially uniform temperature and composition throu hout its mass at the point of extrusion. Prior to extrusion from the vessel, the mixture is brorght to a temperature such that the normally gaseous agent is in non-gaseous form at the applied pressure.

It may be mentioned that agitation of the pressurized mixture of the thermoplastic polymer and a normally gaseous agent in a direction transverse to the flow of the mixture through the mixing and cooling vessel has an effect of adding heat to the mixture by transfer of a part of the mechanical ener y employed to drive the roll into heat energy by the shearing action on the gel or solution. Accordingly, agitation of the mixture to an extreme degree may result in a rise in temperature of the gel or solution, i. e. heat may be added to the mixture by mechanical agitation faster than it can be removed by transfer through surfaces in contact therewith and in contact with a cooling medium. In general, a degree of stirring or agitation transverse to the direction of flow of the mixture through the mixing and cooling zone sufiicient to prevent channeling through said zone, or to avoid uneven distribution of the normally gaseous agent throughout the mass of the mixture, is satisfactory. The degree of agitation recuired to blend the mixture to a homogeneous solution While bringing the same to the desired temperature throughout its mass at the point of extrusion will, of course, vary widely, depending paratus used.

"The normally gaseous-agent to be employed" in the process for forming the cellular product should-be-a substanoeuwhich is soluble in,- orwat. least"*appreciably soluble in; the polymermnder an'applied pressureof several atmospheres Said agent is-one which is gaseous at room'tempera ture and atmospheric ressure: Examples of: substances" which are suitable in v forming the cellular thermoplastic products-are methyl chlo ride; methyl ether, ethyl chloride dichlorodi' fluoromethane, and normally gaseous olefines such as propyleneor butylene; Mixtures of any two-or=n1ore of the above compounds may also The amount of the-normally gaseous agent dissolved in the polymer in forming the cellulan product should 'be such that upon release of the pressure it is vaporized-almost completely, causing not only swelling and expansion-of t-he poly mer to form the cellular product, but at the same time-cool it to a temperature such that it is dimensionally stable. When the thermoplastic polymer is polystyrene and the gel prior to release of the pressure isat a temperature above 85 0., an amount of the normally gaseousiagent sufficient to expand and cool the polymen'tova temperature of; 85C., or lower, :is employed. .Ilhe normally gaseous agent is usually C'.,Linto admixture with the normally gaseous:

agentyin the' desired proportions, at ;superat-.

mospheric pressure in the mixing and cooling zone or 'vessel;. The resulting plastic. mixture.

iSiblBIlflGd under pressure and is brought to a temperature 'suchithat the normally gaseous agent,

is 11115 non-gaseous form at thelpressure zthus ap plied to the mixture.

square inch up. tothe'limits of the apparatus,

buti ia' pressure of from 400 to 2000 pounds persquare :inch ispreferred. In general, a pressure corresponding to the vapor pressure of the normally gaseous agent at the temperature of the gel just prior to extrusion; is satisfactory, although best results .are obtainedwhen somewhat higher pressures are employedhe-solution o gel is formed under pressure. and is brought to atemperature below the critical temperatureof the normally gaseousagent used,,

which. in the case of methyl chloride is. about 1143 C. The gelis suitably brought to a temperature within the range. of from ,60" to 13 c preierably from 90? to 110 0., just prior to release offthe pressure by extrusion of the gel.

into :a. vzone of lower pressure.

In-producing a cellular thermoplastic product by the present method, the heat-plastified, or molten,.polyrner is fed under pressure by means of'theplastics extruderfi, or'in otherusua'l mannor, e; g. by means of a plastics gearpump, into themixing and cooling device I! via inlet Hi;

Anormally gaseous agent capable ofbei-ng dissolved by the polymer is suitably introduced via.

inletlZor inlet" 8 at superatmospheric pressure,

in the desired proportion. The mixture flows through the annular passageway" between the; barrel of the vessel II and the roll l; bypres The pressurexto -be=em-' ployed may vary-from about 300 pounds per sure cfthe screw on the feed 1 materialsentei'mm admixture with each other. The mixture, is .iaigie" tated" transversely to the -flow thereofrby: they rotating roll it and its projecting lugs [6 which rapidlyand :uniformly blends the plastic mixture to'a homogeneous composition. Simultaneously with the blending operation, the. mixture, is brought to a desired ten'iperatureat which it: remains flowable and such that the normally gaseouslagentisin non-gaseous form at thepressure thus applied to the mixture by passing, a cooling medium, e. g. water, through chamber ii and roll. I5, andiorrns a mobile gel .orsol-ution having asubstantially uniform composition and a temperature throughout its cross-section. or; mass at the pointof extrusion. Themobile gel or--sol-ution-is, dischargedor flows from vessel, [1, as-ituis; formed through outlet la and extrusion die .Or orifice Minto a zone of lower, pressure, e. g. into the atmosphere, and swells orexpandsy byqvapor-ization of the normally gaseousagwt; to dorm a cellular mass of the polymer.

It is-important that the gel be. blendedtoJa. homogeneous composition and a substantially lmiforrn temperature throughout its mass, just. prior to release of the pressure, in order to ob,- taina cellular product composedoof individual; cells.ofsubstantiallythe same size. It may be, mentioned that the size of the cells formed in, the cellular product and also the bulk density of-the latter may be varied at Willby varyingthe, temperature .of the gel audits content of the .dis,-, solved volatile agent throughout. its mass .atthe point. of extrusion. In general, an increase. in such; temperature. causes a decrease, in.- the. bulk. density of the product formedv upon release of, thevpressure for a given proportion of. the (1151 solved volatile agent,

T-hepressure maintained on the mixture may be variedeatwill. by varying the size of. the exe trusiondie, orthe lengthof the orifice, through, which the mobilegel is discharged from them'iX ing and cooling vessel, or by changing? the rates effecd of the normally gaseous agent and the polymer tothe vessel. It may bementioned that; in generalthe mobile gel discharged from. the, vessel is a more readily flowable composition than .isthe heat-plastified or molteri polymerl fed into the vessel. Accordingly, an extrusion. dicor orifice having a cross-sectional area con" siderably less than the cross-sectional area of the ifeedinlet .to the vessel is usuallyemployed in order to maintain the, mixture under the .desired pressure during the blending, and cool'mg. operation. Also, for an extrusion die having, a. passagewayacf a given cross-sectional area thereaisrar minimum rate of feed :of the polymer and: the normally gaseous agent to the vessel} which is required in order to obtain a desired pressurelon the mixture. Although the pressure i maintained on the mixture in themixing and, cooling vesselis dependent to a. lesser degree; upon variable oonclitionssuch as the kind and proportion of the normally gaseous agent dise solved in the polymer, and the temperature prior to lextrusionpor release of the pressure on the. geLthepressure can readily be controlled by, regulating the relative rates of feed of the com-- ponents into. the, vessel and discharge ofthe, mobile :gel therefrom throughv a constricted passageway or an. extrusion die as just mentioned; The process :perznits ready change in thepropore. tionstof' the normally gaseous agent and: ma polymer and ,also in the temperatureandiimiessureawem'ployed. Thef -method permitscthe lgpmwrl An arrangement of apparatus consisting of a plastics extruder, having a bore of 3.5 inches and a barrel four feet long surrounded by a divided jacket, in combination with a mixing and cooling device similar to that illustrated in the drawing, was employed to prepare a cellular thermoplastic product from polystyrene. The screw of the plastics extruder had a sealing plate integral therewith, 22 inches from the hopper or feed end of the extruder barrel. The sealing plate contained thirty-six 0.125 inch diameter drill holes equally spaced on a 3 inch circle and was of a diameter enough smaller than the bore of the barrel of the extruder to leave a clearance of 0.020 inch between the two members. The plate formed a plastic seal against countercurrent flow of gas through the extruder barrel. Beyond the plate, in the extruder barrel was 9.

- 0.5 inch diameter inlet for feed of the normally gaseous agent into admixture with the molten polymer. The discharge head of the plastics extruder was connected to an inlet on the mixing and cooling device by a 2 inch diameter conduit 5 feet long. The mixing and cooling device consisted of a jacketed pressure resistant cylindrical barrel having an internal diameter of 4 inches by 32 inches long, surrounded by a chamher with suitable inlets for passage of a heating or cooling medium therethrough. A cylindrical roll 3.5 inches in diameter was mounted within the barrel, with running clearance between the ends of the barrel of the mixer and cooler and the ends of the roll. On its outer surface the roll had eight rows of one-quarter by one-half inch long, lugs, or projections, adapted to scrape the plastic mixture from the barrel and agitate or mix the same when the roll was rotated. An outlet consisting of a 2 inch diameter by one foot long conduit near the discharge end of the mixer and cooler barrel was connected to an extrusion die containing a circular extrusion orifice having a diameter of 0.45 inch.

A mixture consisting of 100 parts by weight of granular polystyrene, 0.75 of Silene (precipitated. calcium silicate) and 0.08 part of barium stearate was fed into the 3.5 inch extruder, from or through the hopper, at a rate of 26.0 pounds per hour, wherein it was compressed by the screw action and was heated to a temperature of 195 C. by passing steam through the surrounding jacket. After being advanced by the screw, beyond the sealing plate in the barrel of the extruder, the heated polymer was mixed with a stream of propylene in amount corresponding to a flow of 1.95 pounds of the propylene per hour. The mixture was cooled as it was advanced by the screw toward the discharge end of the extruder by passing cooling water through the surrounding jacket. The mixture was discharged from the extruder at a temperature of 125 C. and was fed into the mixing and cooling vessel under a pressure of 1100 pounds per square inch. The plastic mixture was advanced, or caused to flow, through the annular passageway between the roll and the barrel of the mixing and cooling vessel by pressure of the screw on the feed materials. The roll was rotated at a speed of 16 revolutions per minute. Cooling Water was passed at a temperature of 60 G. into the outer jacket of the mixing and cooling vessel and flowed therefrom at a temperature of 73 C. The mixture was discharged from the mixing and cooling vessel under a pressure of 780 pounds per square inch as a homogeneous mobile gel having a uniform temperature of 102.5 C. throughout its mass at the point of discharge and was extruded through the 0.45 diameter orifice into the atmosphere. The blending time was approximately 15 minutes. By blending time is meant the time required for an infinitestimal portion of the plastic mixture to traverse the mixing and cooling zone wherein it is mechanically agitated by the roll from the point of feed to said zone to the point of discharge. The mobile gel expanded on release of the pressure to form a substantially round cellular log or bar having a diameter of 2.5 inches. The cellular product had a bulk density corresponding to 1,87 pounds per cubic foot.

Example 2 By procedure similar to that described in Example 1, the granular polystyrene mixture was fed at a rate of 27 .3 pounds per hour into the ex' truder wherein it was heated to a temperature of 190 C. and was mixed with ethyl chloride in amount corresponding to a flow of 2.46 pounds of the ethyl chloride per hour. The mixture was discharged from the extruder at a temperature of 136 C. and was fed into the mixing and cooling vessel under a pressure of 950 pounds per square inch. The roll was rotated at 16 revolutions per minute. Cooling water at a temperature of 78 C. was passed into the jacket of the vessel and flowed therefrom at a temperature of C. The mixture was discharged from the mixing and cooling vessel under a pressure of 610 pounds per square inch as a homogeneous solution having a uniform temperature of 102 C. throughout its mass at the point of discharge and was extruded through a 0.45 inch diameter nozzle into the atmosphere. The gel expanded to form a uniform cellular billet or rod having a diameter of about 2 inches. The cellular product had a bulk density corresponding to 2.04 pounds per cubic foot.

Example 3 The granular polystyrene mixture of Example 1 was fed at a rate of 30 pounds per hour into the extruder described in Example 1, wherein it was heated to a temperature of 191 C. and was mixed with methyl chloride in amount corresponding to a flow of 2.94 pounds of the methyl chloride per hour. The mixture was discharged from the extruder under a pressure of 1050 pounds per square inch and was fed at a temperature of 112.5 C. into the mixing and cooling vessel. The roll was rotated at 16 revolutions per minute. Cooling water was passed at a temperature of 73 G. into the jacket of the vessel and flowed therefrom at a temperature of 77 C. The mixture was discharged from the mixing and cooling vessel under a pressure of 550 pounds as a homogeneous solution having a temperature of 101 0., into the atmosphere through a 0.45 diameter extrusion orifice and expanded to form a cellular product. The product had a bulk density corresponding to 1.65 pounds per cubic foot.

Examples! 'JThe experiment of'Example 3 wasrepeated.

{by feeding the polystyrene mixture at a rate of 29.6 poundsper hour into. admixture with methyl chloride in amount corresponding to a flow of 1:12.87 pounds of the methyl chloride per hour, ,except that the roll was rotated at 31 revolutions '.per minute and the mobile gel was discharged :(rom the mixing and cooling vessel ata tempera- ".ture-of 105C. through the. 0.45inch diameter orifice into the atmosphere. uct had a bulk density corresponding to 1.38 pounds per cubic foot.

The cellular. prod- Erample. 5

"The experiment was repeated, except-that the roll wasrotated at 8 revolutions per minute and "the mobile gel was dischargedfrom the mixing and cooling vessel under a pressure of 700 pounds per square inch and at .a temperature of- 94 C.

'Thecellularproduct had a bulk density correspending to 2.10 pounds per cubic foot.

Example 6 By procedure similar to that described in Example 1, the granular polystyrene mixture was fed at a rate of 27.3 pounds per hour into the extruder wherein it was heated to a temperature of 193C. and was mixed with methyl chloridein amount-corresponding to a flow of 1.62 pounds of the methyl chloride per hour. The mixture was discharged from the extruder at a temperature of 141 C. and was fed into the mixing and cooling vessel under a pressure of 1000 *pounds-per square inch. The roll was rotated *at 16 revolutions per minute. Cooling water was atmosphere as it was formed and expanded to form a cellular product. having a diameter of 2.25

inches. The cellular product had a bulk. density of 1.86 pounds per cubic foot.

Example 7 The granular polystyrene mixture of Example 1 was fed at a rate of 31.2 pounds per hour into the extruder described in Example 1, wherein it was compressed and heated to a temperature of 198 C. A liquid mixture consisting of '75 per cent by weight of propylene and 25 per cent of methyl chloride was fed at a rate corresponding to a flow of 2.16 pounds of the mixture per hour into admixture with the molten polystyrene under apressure of 2000 pounds per square inch. The resulting mixture was discharged from the extruder at a temperature of 111 C. and was fed into the mixing and cooling vessel under a pressure of 1750 pounds per square inch. iihe roll was rotated at 16 revolutions per minute. Cooling water atatemperature of 48 C. was passed into the jacket of the mixing and cooling vessel and flowed therefrom at a temperature of 56 C. The blended mixture flowed from the mixing and cooling vessel under a pressure of 710 pounds per square inch as a uniform solution having a temperature of 101 C. through a 0A5 inch diameter extrusion orifice into the atmosphere and expanded as it was formed to produce a cellular product. The cellular product had a bulk density of 2.09 pounds per cubic foot.

10 Ex mp By procedure similar to that describedin Example 1, the g-ranu-larpol-ystyrene mixture was fed at a rate of 28.1 pounds per hour into the extruder whereinit was heated to a temperature of C. and was mixed with methyl chloride in amount corresponding to a flow of 3.93 pounds of the methyl chloride per-hour. The resulting mixture was discharged from the extruder at a temperature of 113C. and Wasted intothe mixing and cooling vessel under a pressure of 600 pounds per square inch. The roll was rotated at 16 revolutions per minute. "Cooling water was passed into "the jacket of the mixing and cooling vessel at a temperature of 79 C. and flowed therefrom at a temperatureof 98"C.. The blended mixture Example 9 The. granular polystyrene mixture of Example 1 wasfed in continuous. manner at a rate of 28.1

pounds per hour into the plastics extruder described in Example 1,. wherein it was heated to a temperature of 188C. and was advanced through the barrel of the extruder by the screw. After being advanced byzthe screw beyond the sealing plate, the molten polymer was mixed with Freon-.12 .(dichlorodifluoromethane) in amount corresponding to aqfipwof 6.11 pounds of the dichlorodifluoromethane .per hour. The mixture wasdischalieed fromthe extruder at a temperature 01122. CQJQ-Bd was fed under a pressure of 1100 pounds per square inch into the barrel of the mixing and. cooling vessel. The roll was rotated at lfi revolutions. per minute. Cooling water at sel under apressure of .700 pounds per square .inch as a uniform solution having a temperature of 103 C. and was. extruded through a 0.45. inch diameter extrusion orifice into the atmosphere. The solution expanded upon release of the pressure to form a stable cellular rod or bar. The cellular product had a bulk density of 1.66 pounds per cubic foot and was composed for the most part of substantially uniform cells having a diameter of approximately one millimeter.

The method herein disclosed may be applied in producing uniform cellular products from other thermoplastic resins such as the solid polymers of para chlorostyrene, meta chlorostyrene, ortho-methylstyrene, meta-methylstyrene, paramethylstyrene, meta-ethylstyrene, ortho-ethylstyrene, para-isopropylstyrene, methyl methacrylate and vinyl acetate and from solid thermoplastic copolymers such as those of styrene and methyl methacrylate, styrene and alpha-methylstyrene, or of vinyl acetate and vinyl chloride. In all instances, it is important that the aforementioned limitations with regard to the proportion of a volatile agent employed in forming a gel of the polymer by agitating under pressure and the temperature and pressure of the gel prior to release of the pressure he observed.

Other modes of applying the principle of the invention may be employed instead of those explained, change being made as regards the steps I l or compounds herein employed, provided the steps or compounds stated in any of the following claims or the equivalent of such steps or compounds be employed.

We claim:

1. A method of making a cellular thermoplastic article which comprises feeding a heat-plastified thermoplastic polymer at a temperature between 120 and 220 C. and a normally gaseous agent capable of being dissolved by the polymer into admixture with each other under pressure in amounts corresponding to from 0.05 to 0.25 part by weight of the normally gaseous agent per part of the polymer, to a mixing and cooling zone wherein the mixture is caused to flow through said zone solely by pressure of the feed thereto against frictional resistance to flow upon the mass against walls defining said zone, continuously subjecting the mixture to a non-forwarding agitation transverse to the flow thereof through said zone at each of a plurality of closely spaced regions throughout a major portion of the length of the zone in a manner preventing appreciable accumulation of the mixture as a static layer on stationary walls defining said zone, the transverse agitation being sufficient to prevent channeling and to blend the mixture as a substantially uniform composition, While cooling the mixture to a temperature at which it remains flowable and below the boiling point of the normally gaseous agent at the pressure thus applied to the mixture, to form a homogeneous solution having a substantially uniform temperature of from 60 to 130 C. throughout its mass at the point of discharge from said zone and continuously extruding the solution into a zone of sufiiciently lower pressure to cause expansion of the extruded material with resultant formation of a cellular plastic body.

2. A method of making a cellular thermoplastic article which comprises feeding a heat-plastiiied thermoplastic polymer at a temperature between 160 and 200 C. and a normally gaseous agent capable of being dissolved by the polymer into admixture with each other under pressure, in amounts corresponding to from 0.05 to 020 part by weight of the normally gaseous agent per part of the polymer, to a mixing and cooling zone wherein the mixture is caused to flow through said zone solely by pressure of the feed thereto against frictional resistance to flow upon the mass against walls defining said zone, continuously subjecting the mixture to a non-forwarding agitation transverse to the flow thereof through said zone at each of a plurality of closely spaced regions throughout a major portion of the length of the zone in a manner preventing appreciable accumulation of the mixture as a static layer on stationary walls defining said zone, the transverse agitation being sui'ficient to prevent channeling and to blend the mixture as a substantially uniform composition, while cooling the mixture to a temperature at which it remains flowable and below the boiling point of the normally gaseous agent at the pressure thus applied to the mixture, to form a homogeneous solution having a substantially uniform temperature of from to C. throughout its mass at the point of discharge from said zone and continuously extruding the solution into a zone of sufiiciently lower pressure to cause expansion of the extruded material with resultant formation of a cellular plastic body.

3. A method as claimed in claim 2 wherein the thermoplastic polymer is a normally solid polystyrene.

l. A method as claimed in claim 3 wherein the normally gaseous agent is methyl chloride.

5. A method as claimed in claim 3 wherein the normally gaseous agent is propylene.

6. A method as claimed in claim 3 wherein the normally gaseous agent is ethyl chloride.

7. A method as claimed in claim 3 wherein the normally gaseous agent is a mixture consisting of methyl chloride and propylene.

JOHN LLOYD MCCURDY. CHANCEY EDVIARD DE LONG.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,333,786 Hessen Nov. 9, 1943 2,354,260 Haney et al July 25, 1944 2,450,436 McIntire Oct. 5, 1948 2,515,250 Mclntire July 18, 1950 

